Add option to model meta class to prevent proxy models being polymorphic

[bckohan]

Fixes #376 #390

I need to rebase and didn't have permissions to push to the original branch for #544, so this replaces that PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.55%. Comparing base (0d820d0) to head (4e2825d).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #676      +/-   ##
==========================================
+ Coverage   75.40%   75.55%   +0.14%     
==========================================
  Files          21       21              
  Lines        1342     1350       +8     
  Branches      211      214       +3     
==========================================
+ Hits         1012     1020       +8     
  Misses        257      257              
  Partials       73       73              

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[Copilot]

Pull request overview

This PR adds a new polymorphic__proxy meta option to control whether proxy models behave as traditional Django proxies (aliases) or as polymorphic proxies (distinct types). This addresses issues #376 and #390 by allowing developers to opt-in to traditional Django proxy behavior when needed.

Key Changes

• Introduced polymorphic__proxy meta attribute that can be set to True (traditional Django proxy/alias) or False (polymorphic proxy with distinct type)
• Modified the manager's queryset logic to respect the new attribute when applying instance_of filters
• Added comprehensive test coverage with multiple proxy model variants to validate the new behavior

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
src/polymorphic/base.py	Added metaclass logic to extract and store polymorphic__proxy from model Meta, with default based on proxy setting
src/polymorphic/managers.py	Changed queryset logic from checking _meta.proxy to checking _meta.polymorphic__proxy to control instance filtering
src/polymorphic/tests/models.py	Added 6 new test model classes demonstrating different proxy configurations and inheritance hierarchies
src/polymorphic/tests/test_orm.py	Added 6 new test cases to validate polymorphic proxy behavior, traditional Django proxy behavior, and type conversion scenarios
src/polymorphic/tests/migrations/0001_initial.py	Generated migration file for the new test models

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